1. Field of the Invention
The present invention relates to a rectifier system for a generator of a motor vehicle.
2. Description of Related Art
In the case of three-phase generators or alternators in motor vehicles in particular, B6 a.c. bridges are usually used as rectifiers. In this connection, 6 pn-semiconductor diodes made from silicon are used as rectifying elements. FIG. 1 shows an example of such a system.
Occasionally, a.c. bridges having more than 6 diodes are also used. For example, bridges having diodes connected in parallel are used in the case of very high currents. A possible system is shown as a circuit diagram in FIG. 2.
The semiconductor diodes are usually designed as Zener diodes. This case is assumed for the two depictions in the circuit diagram. In addition to a three-phase generator and the a.c. bridge, a complete generator system also includes a voltage regulator which ensures that the rectified voltage has a specific value, for example, 14.3 V. The voltage regulator controls the excitation current of the generator. If the generator voltage exceeds the intended value, the regulator interrupts the excitation current. The excitation becomes weaker and the generator voltage drops. If the generator voltage drops below a lower setpoint value, the regulator switches the excitation current on again. The excitation current is fed to the excitation winding of the generator via sliprings. Generators having multifunction regulators draw the excitation current directly from positive terminal B+. The elements of the voltage regulator are mostly integrated monolithically into silicon.
In the starting case, the regulator switches on a pre-excitation current supplied by the battery. As soon as the generator's rotor rotates, the regulator detects a voltage signal Up on a phase connection. From its frequency, the regulator is able to derive the generator speed. When a set switch-on speed is reached, the regulator switches on the full excitation current, so that the generator begins to deliver current.
In order for the voltage regulator to detect the speed of the rotor, the alternating current portion of phase signal Up_AC must have a specific minimum voltage. In addition, direct current portion Up_DC of the phase signal may usually not exceed a certain threshold, since the evaluation circuit of the regulator frequently functions only up to a specific voltage value. Thus, minimally necessary phase alternating voltage Up_AC may be, for example, 3 volts, measured from peak to peak, and maximally occurring direct current portion Up_DC may be 8 volts.
During the pre-excitation clocking, if the generator is not yet delivering current into the electrical system, or if no rectification is taking place yet, the direct current value and the alternating voltage value of the phase voltage are influenced by the reverse currents of the rectifier diodes. In this operating state, battery voltage UB is present at terminal B+. If the same reverse currents flow through the diodes (series connection), phase voltage Up_DC=UB/2.
In this connection, it was assumed that the internal resistance of the regulator between the phase tap and ground is infinite. This is ordinarily not the case, but instead the regulator has a resistance of, for example, 1-100 kΩ between the phase tap and ground. This resistor is, for example, connected to diode D3 in parallel in the example according to FIG. 1. However, its influence will be disregarded from further consideration for reasons of clarity. The curve of reverse current IR of a pn-type diode made of silicon is extremely temperature-dependent and increases as the reverse voltage rises. Greater detail in this regard may be found in S. M. Sze, “Physics of Semiconductor Devices,” John Wiley & Sons, New York, 1981, p. 91. This is also shown in FIG. 3.
Since positive and negative diodes are each connected in series, phase voltage Up_DC changes if the positive and negative diodes are located at positions of varying temperature. In the case of high reverse currents of the positive diode, direct current portion Up_DC of the phase voltage may assume values above the maximum permissible threshold in the extreme case. The result of this is that the regulator does not detect the start speed or detects it too late and does not activate the generator or activates it only at higher speeds.
A disadvantage is that varying reverse currents of the pn-type diodes, caused, for example, by varying diode temperatures, may cause direct current portion Up_DC of the phase voltage to exceed the maximum permissible value and that the starting behavior of the generator is no longer ensured.
A generator system for a vehicle is known from U.S. Pat. No. 7,084,610 B2, in which a B6 bridge made up of Schottky diodes is used as a rectifier bridge. A Zener diode connected in the reverse direction is used for voltage limitation. It thus limits the operating voltage in relation to interferences and interference impulses originating from the generator.